The planet appears to be at the early stages of its sixth mass extinction, and humans are responsible, a new study finds.

Published last week in the journal Science, the study incorporates scientific literature review and data analysis by a team of international scientists, led by author Rodolfo Dirzo, a professor of biology at Stanford.

Scientists say that the Earth has already sustained five mass species die-offs, the most recent having occurred millions of years ago. But unlike the mass extinction events prior, the one likely underway is caused by human-made climate change and habitat loss.

Over 320 vertebrates have gone extinct since 1500, and today, between 16 and 33 percent of vertebrates are endangered or threatened, the study notes.

Large vertebrate animals (megafauna), which include elephants, zebra, and polar bears, face the steepest decline, because they require large habitats and are targeted by human hunters for their more plentiful meat.

The loss of this megafauna throws ecosystems off-kilter, leading to problems like rodent infestations that then impact the well-being and stability of a broad swath of animal species—including humans.

According to the study, the especially steep fall of megafauna is a characteristic of prior mass extinction events.

Animals without backbones are also showing signs of decline. Over the past 35 years, the number of invertebrate animals has decreased by 45 percent as the human population has increased two-fold. This ripples throughout ecosystems. Falls in beetles, butterflies, worms, and others impact crops growth, the decomposition of organic substances, and more.

The authors conclude that the trends point to another mass die-off of terrestrial life.

“We tend to think about extinction as loss of a species from the face of Earth, and that’s very important, but there’s a loss of critical ecosystem functioning in which animals play a central role that we need to pay attention to as well,” Dirzo said in a press statement. “Ironically, we have long considered that defaunation is a cryptic phenomenon, but I think we will end up with a situation that is non-cryptic because of the increasingly obvious consequences to the planet and to human wellbeing.”